This invention relates to a proton-conducting membrane, method for producing the same, and fuel cell using the same, more particularly the proton-conducting membrane, excellent in resistance to heat and durability and showing excellent proton conductivity at high temperature, method for producing the same, and fuel cell using the same.
Recently, fuel cell has been attracting attention as a power generating device of the next generation, which can contribute to solution of the problems related to environments and energy, now having been increasingly becoming serious social problems, because of its high power generation efficiency and compatibility with the environments.
Fuel cells fall into several categories by electrolyte type. Of these, a polymer electrolyte fuel cell (PEFC), being more compact and generating higher output than any other type, is considered to be a leading fuel cell type in the future for various purposes, e.g., small-size on-site facilities, and as movable (i.e., power source of vehicles) and portable cells.
However, PEFCs are still in the development or testing stages and not yet commercialized so far, in spite of their inherent advantages in principle, because of lack of the practical electrolytic membrane which satisfies all of the requirements, e.g., resistance to heat, durability and proton conductivity. The electrolytic membranes for the current PEFCs are mainly of fluorine-based ones, with a perfluoroalkylene as the main skeleton, and partly with ion-exchangeable groups, e.g., sulfonic and carboxylic acid groups, at the terminal of the perfluorovinyl ether side chains. Several types of these fluorine-based membranes have been proposed, e.g., Nafion membrane (Du Pont, U.S. Pat. No. 4,330,654), Dow membrane (Dow Chemical, Japanese Patent Application Laid-Open No.4-366137), Aciplex membrane (Asahi Chemical Industry, Japanese Patent Application Laid-Open No.6-342665), and Flemion membrane (Asahi Glass).
The current PEFCs using the above fluorine-based membranes as the electrolyte are normally operated in a relatively low temperature range, e.g., room temperature to around 80xc2x0 C., because the fluorine-based membrane itself has a glass transition temperature (Tg) of around 130xc2x0 C., above which its ion channel structure responsible for the ion conductivity will be destroyed. It is not desirable for a fuel cell to operate in a low temperature range, because of some serious problems, e.g., low power generation efficiency and notable poisoning of the catalyst with carbon monoxide.
Fuel cells have been continuously developed to operate in a higher temperature range, in order to avoid the problems resulting from operation in a low temperature range. Operability at higher temperature brings about several advantages. For example, when operated at 100xc2x0 C. or higher, power generation efficiency should increase and, at the same time, heat can be utilized to improve energy efficiency. When operating temperature can be increased to 140xc2x0 C., still other advantages, in addition to the above, can be expected, e.g., increased choices for the catalyst material, thus helping reduce fuel cell cost.
A variety of electrolyte membranes (e.g., proton-conducting membranes) have been proposed so far to increase operating temperature of PEFCs.
Some of more representative ones are heat-resistant aromatic-based polymers to replace the conventional fluorine-based membranes. These include polybenzimidazole (Japanese Patent Application Laid-Open No.9-110982), polyether sulfone (Japanese Patent Application Laid-Open Nos.10-21943 and 10-45913), and polyetheretherketone (Japanese Patent Application Laid-Open No.9-87510). However, each of these aromatic-based polymers is highly rigid, possibly causing damages while the membrane-electrode assembly (MEA) is formed.
They have other types of disadvantages. For example, they are modified with an acidic group (e.g., sulfonic or phosphoric acid group) to have proton conductivity necessary for the electrolytic membrane, with the result that they are water-soluble or swelling in the presence of water. The water-soluble ones are not applicable to fuel cells, because water is produced therein. On the other hand, those swelling in the presence of water may cause problems, because the swelling can generate a sufficient stress in the membrane to damage the electrode, or deteriorate membrane strength leading to its destruction.
On the other hand, the following inorganic materials have been proposed as the proton-conducting materials. For example, Minami et al. incorporate a variety of acids in hydrolysable silyl compounds to prepare inorganic proton-conducting materials (Solid State Ionics, 74 (1994), pp.105). They stably show proton conductivity at high temperature, but involve several problems; e.g., they tend to be cracked when made into a thin film, and difficult to handle and make them into MEAs. Several methods have been proposed to overcome these problems. For example, the proton-conducting material is crushed to be mixed with an elastomer (Japanese Patent Application Laid-Open No.8-249923) or with a polymer containing sulfone group (Japanese Patent Application Laid-Open No.10-69817). However, these methods have their own problems. For example, the polymer as the binder for each of these methods has no bond or the like with an inorganic crosslinked compound and has basic thermal properties not much different from those of the polymer itself, with the result that it undergoes structural changes in a high temperature range, failing to stably exhibit proton conductivity.
A number of R and D efforts have been made for various electrolyte membranes to solve these problems involved in the conventional PEFCs. None of them, however, have succeeded in developing proton-conducting membranes showing sufficient durability at high temperature (e.g., 100xc2x0 C. or higher) and satisfying the mechanical requirements.
It is an object of the present invention to provide a proton-conducting membrane excellent in resistance to heat and durability and showing excellent proton conductivity at high temperature, which can solve the problems involved in the conventional PEFCs, and a method for producing the same and fuel cell using the same.
The inventors of the present invention have found, after having extensively studied a variety of electrolyte membranes to solve the above problems, that an innovative organic/inorganic composite membrane can be obtained by including, as the essential components, a selected combination of specific organic material, three-dimensionally crossliiked structure containing a specific metal-oxygen bond, agent for imparting proton conductivity and specific proton-conducting material, reaching the present invention. It shows much higher resistance to heat and durability, and proton conductivity at high temperature than the conventional one, because of the covalent bond formed between the organic material and three-dimensionally crosslinked structure to disperse them very finely at the molecular level (nano-dispersion).
The first invention is a proton-conducting membrane, comprising (A) an organic material, (B) a three-dimensionally crosslinked structure containing a specific metal-oxygen bond, (C) an agent for imparting proton conductivity, and (D) water, wherein
(i) the organic material (A) has a number-average molecular weight of 56 to 30,000, and at least 4 carbon atoms connected in series in the main chain, and
(ii) the organic material (A) and three-dimensionally crosslinked structure (B) are bound to each other via a covalent bond.
The second invention is the proton-conducting membrane of the first invention, wherein the organic material (A) is a polyether.
The third invention is the proton-conducting membrane of the second invention, wherein the organic material (A) is a polytetramethylene oxide.
The fourth invention is the proton-conducting membrane of the first invention, wherein the organic material (A) is a polymethylene.
The fifth invention is the proton-conducting membrane of the fourth invention, wherein the organic material (A) is octamethylene.
The sixth invention is the proton-conducting membrane of the first invention, wherein the organic material (A) contains a water-retentive resin (E) having less than 4 carbon atoms connected in series in the chain.
The seventh invention is the proton-conducting membrane of the sixth invention, wherein the water-retentive resin (E) is a polyethylene oxide.
The eighth invention is the proton-conducting membrane of the first invention, wherein the organic material (A) is a mixture of polytetramethylene oxide and polyethylene oxide.
The ninth invention is the proton-conducting membrane of the first invention, wherein the three-dimensionally crosslinked structure (B) is formed by a silicon-oxygen bond.
The tenth invention is the proton-conducting membrane of the first invention, wherein the agent (C) for imparting proton conductivity is an inorganic solid acid.
The 11th invention is the proton-conducting membrane of the tenth invention, wherein the inorganic solid acid is tungstophosphoric acid. The 12th invention is the proton-conducting membrane of the first invention, which contains 5 to 500 wt. parts of the agent (C) for imparting proton conductivity per 100 wt. parts of the organic material (A) and three-dimensionally crosslinked structure (B) totaled.
The 13th invention is the proton-conducting membrane of the first invention, which contains water (D) at 1 to 60 wt. %, based on the whole proton-conducting membrane.
The 14th invention is the proton-conducting membrane of the first invention, which further contains a reinforcing agent (F).
The 15th invention is the proton-conducting membrane of the 14th invention, wherein the reinforcing agent (F) is glass fibers.
The 16th invention is a method for producing a proton-conducting membrane, comprising steps of preparing a reaction system containing a mixture of an organic material (A), hydrolyzable inorganic compound which forms a three-dimensionally crosslinked structure (B) and agent (C) for imparting proton conductivity; forming the reaction system into a film; and sol-gel reaction of the film in the presence of water vapor or liquid water (D), to form the three-dimensionally crosslinked structure (B) by the metal-oxygen bond in the film. The 17th invention is a method for producing a proton-conducting membrane, comprising steps of preparing a reaction system containing a mixture of an organic material (A), hydrolyzable inorganic compound which forms a three-dimensionally crosslinked structure (B) and agent (C) for imparting proton conductivity; forming the reaction system into a film; and sol-gel reaction of the film in the presence of water vapor or liquid water (D) and vapor or liquid of an alcohol having a carbon number of 4 or less, to form the three-dimensionally crosslinked structure (B) by the metal-oxygen bond in the film.
The 18th invention is the method for producing a proton-conducting membrane of the 16th or 17th invention, wherein the organic material (A) and hydrolyzable inorganic compound which forms the three-dimensionally crosslinked structure (B) are mixed with each other in an organic solvent (G).
The 19th invention is the method for producing a proton-conducting membrane of the 18th invention, wherein the organic solvent (G) is further incorporated with a compound (H) having a relative dielectric constant of 20 or more and boiling point of 100xc2x0 C. or higher.
The 20th invention is the method for producing a proton-conducting membrane of the 19th invention, wherein the compound (H) having a relative dielectric constant of 20 or more and boiling point of 100xc2x0 C. or higher is selected from the group consisting of ethylene carbonate, propylene carbonate and butylene carbonate.
The 21st invention is a fuel cell which uses the proton-conducting membrane of one of the first to 15th inventions.